JP3426496B2 - High strength long life carburizing steel excellent in delayed fracture resistance and method of manufacturing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to hydrogen, a heat treatment atmosphere, an environment in use, etc., which are originally contained in steel materials, for applications such as gears, shafts and bearings which are surface-hardened by carburizing or carbonitriding. The present invention relates to a high-strength and long-life carburized steel having excellent resistance to delayed fracture that occurs due to hydrogen penetrating from the steel and a manufacturing method thereof.

[0002]

2. Description of the Related Art It is widely known that delayed fracture is generated from hydrogen invading from the outside such as the environment of use under tensile stress and hydrogen contained in steel materials, such as structures such as bolts and bridges. . In these fields, it is often the case that countermeasures are taken from the material side, such as adjusting the hardness of raw materials and alloying elements. Conventionally, delayed fracture has not been a serious problem for gears and shafts that have been surface hardened by carburizing or carbonitriding, but delayed fracture may be a problem for some carburized parts that have bolts. , For example, JP-A-4-29
7550 introduces that the delayed fracture strength can be improved by the combined addition of Mo-V. However, this steel requires the addition of a large amount of Mo and V and is not economical.
Also, it does not mention important characteristics such as rolling life of gears.

[0003]

DISCLOSURE OF THE INVENTION The inventors of the present invention have made intensive studies on the delayed fracture behavior of case-hardening steel, and have found that hydrogen intrusion reduces not only strength but also life characteristics. The object of the present invention is to provide a high-strength carburizing steel having a long life and excellent delayed fracture resistance, which is suitable for gears, shafts, bearings, etc., which are used after carburizing or carbonitriding, by optimizing the chemical composition. It aims to provide it at a low cost by developing a commercialization method and a manufacturing method that makes it possible.

[0004]

Means for Solving the Problems In researching the above-mentioned problems, the inventors have made it possible to trap hydrogen in steel, which causes delayed fracture, by fine Ti-based precipitates,
It was found that it can be invalidated to increase the resistance to delayed fracture and prolong the service life. Further, in this case, it has been found that the smaller the particle size of the precipitate, the stronger the force of trapping hydrogen, and that it is effective to improve the characteristics as uniformly and finely as possible. However, it has been found that simply increasing the amount of addition causes only coarsening of precipitates, the amount of fine precipitates effective as trap sites does not increase, and the hydrogen trapping force weakens as a whole. In addition, coarsened precipitates become a source of stress concentration and adversely affect fatigue strength and life characteristics. It is important to optimize the addition amount and to ensure finer precipitates. It was found that it is necessary to optimize the manufacturing conditions for steel products.

That is, the means for achieving the above object is, in the invention of claim 1, a mass ratio of C: 0.10.
~ 0.40%, Si: 0.05-0.50%, Mn:
0.2 to 2.0%, Ti: 0.05 to 0.20%, A
l: 0.010~0.50%, N: 0.0120 % or less, O: 12 ppm contained the following consists of a balance of Fe and unavoidable impurities, the size is less 70nm in average T
It is a high-strength, long-life carburizing steel having excellent delayed fracture resistance by finely dispersing i carbide and Ti carbonitride in the steel.

According to the second aspect of the invention, the mass ratio of C:
C: 0.10 to 0.40%, Si: 0.05 to 0.50
%, Mn: 0.2 to 2.0%, Ti: 0.05 to 0.2
0%, Al: 0.010 to 0.50%, N: 0.012
0% or less, O: 12 ppm or less, and N
i: 0.1 to 2.0%, Cr: 0.20 to 2.0%, M
o: 1 to 2 selected from 0.05 to 1.0%
High strength with excellent delayed fracture resistance by finely dispersing Ti carbides and Ti carbonitrides containing at least one species and the balance Fe and unavoidable impurities and having an average size of 70 nm or less in steel. It is a long-life carburizing steel.

According to the invention of claim 3, in the high-strength and long-life carburizing steel according to the means of claim 1 or 2, B: 0.0005 to 0.005 as an alloying component in a mass ratio.
It has an excellent delayed fracture resistance characterized by containing 0% of Ti, the balance Fe and unavoidable impurities, and finely dispersing Ti carbide and Ti carbonitride having an average size of 70 nm or less in steel. High strength and long life carburizing steel.

According to the invention of claim 4, in a mass ratio, C:
0.10 to 0.40%, Si: 0.05 to 0.50%,
Mn: 0.2-2.0%, Ti: 0.05-0.20
%, Al: 0.010 to 0.50%, N: 0.0120
% Or less and O: 12 ppm or less, and a steel material composed of the balance Fe and unavoidable impurities is heated and rolled to a temperature range of 1200 to 1350 ° C., and the same temperature or 800
By rolling or forging to a specified steel material or part at a temperature of 1050 ° C, the size is averaged in the subsequent heat treatment process.
Is a finely dispersed Ti carbide or Ti carbonitride having a thickness of 70 nm or less in the steel, and is a method for producing a high-strength long-life carburizing steel having excellent delayed fracture resistance.

According to the invention of claim 5, in a mass ratio, C:
0.10 to 0.40%, Si: 0.05 to 0.50%,
Mn: 0.2-2.0%, Ti: 0.05-0.20
%, Al: 0.010 to 0.50%, N: 0.0120
% Or less, O: 12 ppm or less, and Ni:
0.1-2.0%, Cr: 0.20-2.0%, Mo:
A steel material containing one or more selected from 0.05 to 1.0% and the balance Fe and unavoidable impurities is heated and rolled to a temperature range of 1200 to 1350 ° C.,
Further, by rolling or forging into a predetermined steel material or component at the same temperature or a temperature of 800 to 1050 ° C,
Manufacture of high-strength, long-life carburizing steel having excellent delayed fracture resistance, characterized in that Ti carbide and Ti carbonitride having an average size of 70 nm or less are finely dispersed in the steel in the subsequent heat treatment step. Is the way.

According to the invention of claim 6, in the alloy component of the steel material in the method for producing high strength and long life carburizing steel according to the means of claim 4 or 5, B: 0.005 to mass ratio.
A steel material containing 0.050% and the balance Fe and unavoidable impurities is heated to a temperature range of 1200 to 1350 ° C.
By rolling and further forging or forging into a predetermined steel material or component at the same temperature or at a temperature of 800 to 1050 ° C., Ti carbide or Ti carbonitride having an average size of 70 nm or less is added to the steel in the subsequent heat treatment step. A method for producing a high-strength, long-life carburizing steel having excellent delayed fracture resistance, which is characterized in that it is finely dispersed.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The reasons for limitation in carrying out the present invention will be described below. First, the chemical components (hereinafter,
" Mass ratio". The reason for limitation is described.

Regarding C: 0.10 to 0.40%, C is an essential element for securing the hardenability and the strength of parts, but if it is less than 0.10%, the hardenability is insufficient. Therefore, it is difficult to secure sufficient component strength, so the lower limit is set to 0.1.
0% On the other hand, if it exceeds 0.40%, the strength of the core portion is excessively increased, the toughness is lowered, and unstable fracture is likely to occur, so the upper limit is made 0.40%.

Si: 0.05 to 0.50%, S
i is added to improve the deoxidizer and hardenability, but 0.0
If it is less than 5%, the deoxidizing effect is not sufficient, and if it exceeds 0.50%, the workability deteriorates, so the lower limit is made 0.05% and the upper limit is made 0.50%.

Regarding Mn: 0.2 to 2.0%, Mn is also added to improve the deoxidizing agent and hardenability, like Si, but if it is less than 0.20%, the hardenability is insufficient. If it exceeds 2.0%, the workability is significantly deteriorated, so the lower limit is 0.20.
%, And the upper limit is 2.0%.

For Ti: 0.05 to 0.20%, T
i is finely dispersed in the steel as Ti carbide or Ti carbonitride to improve fatigue characteristics, life characteristics, grain size, and acts as a hydrogen trap site. If it is less than 0.05%, its effect is not sufficient, and if it exceeds 0.20%, Ti carbide or Ti
Since carbonitrides become coarse and do not show sufficient effect,
The lower limit is 0.05% and the upper limit is 0.20%.

Al: 0.005 to 0.50%,
Al is added as a deoxidizing agent, but if it is less than 0.005%, it has no effect, and if it exceeds 0.050%, alumina-based oxide increases and fatigue strength and workability deteriorate, so the lower limit is set. The upper limit is 0.005% and the upper limit is 0.050%.

N: 0.0120% or less, in the case of ordinary case-hardening steel, N is positively added to obtain the grain size refining effect of AlN, but in the steel of the present invention, conversely, the N content is large. As it grows, coarse Ti nitrides and Ti carbonitrides will be precipitated, so it is desirable that the amount is as small as possible. The upper limit is 0.0120% due to the relationship with manufacturing conditions
Below.

O: About 12 ppm or less, O is present in the steel as an oxide-based inclusion and may be considered to function as a hydrogen trap site, but rather as a coarse precipitate,
The role of reducing fatigue strength is large, and it is desirable that it be as small as possible. Therefore, the upper limit is set to 12 ppm.

In addition to the above components, in the steel of the present invention, Ni, C
r and Mo can be contained alone or in combination. These actions are as follows.

Ni: About 0.1 to 2.0%, Ni improves hardenability and improves fatigue strength, toughness and the like.
If it is less than 0.1%, the effect is not sufficient, and if it exceeds 2.0%, it becomes difficult to soften the material and the workability is significantly reduced, so the upper limit is made 2.0%.

Cr: About 0.20 to 2.0%, Cr
Improves hardenability and spheroidization of carbides, and improves fatigue strength and toughness. If it is less than 0.2%, the effect is not sufficient, and if it exceeds 2.0%, it becomes difficult to soften the material and the workability is remarkably lowered, so the upper limit is made 2.0%.

Mo: About 0.05 to 1.0%, Mo
Improves hardenability and improves fatigue strength and toughness.
If less than 0.05%, the effect is not sufficient, 1.0%
If it exceeds, the softening of the material becomes difficult and the workability is significantly lowered. Further, the above effect is saturated and it becomes disadvantageous in terms of cost, so the upper limit is made 1.0%.

Further, in addition to the above components, in the steel of the present invention, B
Can be included. This action is as follows.

B: About 0.0005 to 0.0050%, B is added as an element for improving induction hardenability. If less than 0.0005%, the effect is not sufficient,
If it exceeds 0.0050%, the hot workability is deteriorated, so the upper limit is made 0.0050%.

When the steel material having the above chemical composition is manufactured under the following conditions, the most effective fine Ti-based precipitates are obtained and excellent delayed fracture properties are exhibited.

That is, when the steel melted in the above composition range is rolled into a billet or a steel material, 1200 to 1350
After the final carburizing, quenching and tempering, the size is increased by heating and rolling in the temperature range of ℃, and further rolling or forging into a steel material of a predetermined size in the same temperature range or in the lower temperature range of 800 to 1050 ° C. Ti less than 70 nm on average
Carbides and Ti carbonitrides can be finely dispersed in steel, and high strength and long life bearing steel having excellent delayed fracture resistance can be obtained.

[0027]

[Table 1]

[0028]

Example: 1000 kg of test material having the chemical composition shown in Table 1
Melted in a vacuum melting furnace and in the temperature range of 1200 to 1350 ° C., and part of the same temperature range and 800 to 1050 ° C.
65mmφ and 30mmφ forged further in the temperature range of
Finished. After normalizing, it is processed into a test piece, 930
Hold at ℃ 5.5 hours for oil quenching, then 180 ℃
Then, it was held for 2 hours and air-cooled for carburizing, quenching and tempering to obtain a final use state.

With respect to the obtained steel material, the thrust life characteristics and the delayed fracture characteristics under the normal clean lubrication and the lubrication containing water (hydrogen invading environment) were investigated.

[0030]

[Table 2]

Thrust life test is 60 mm × 40 mm ×
Using a test piece of 8 mm, under normal lubricating oil of spindle # 60 and under lubricating oil containing 0.5% pure water as a hydrogen intrusion atmosphere, the test was performed at HERZ surface pressure: 5292 MPa and stress load rate: 1800 cpm. The 10% probability life (B10 life) was calculated. FIG. 1 schematically shows the structure of the thrust life tester. In FIG. 1, 1 is a test piece holding frame, 2 is a thrust test piece (60φ × 5 to 10 mm), 3
Is a retainer, 4 is a steel ball of 3/8 inch (9.525 mmφ), 5 is an upper race (# 5130 thrust bearing race), 6 is a rotating shaft (1200 rpm), and 7 is lubricating oil (# 60 spindle oil).

For the delayed fracture characteristics, the breaking load was determined by a tensile test after dipping in 5% hydrochloric acid using the notch test piece shown in FIG. Further, the fatigue limit in the atmosphere and in the dropping of pure water was determined by a rotating bending fatigue test using a φ8 mm smooth test piece.

Table 2 shows the production conditions for each test material, the measurement results of the precipitate particle size, and the results of various tests. In addition, here, the rolling conditions in which two conditions are entered indicate that the forging is performed twice.

It can be seen from Table 2 that the steels of the present invention have little deterioration in life characteristics in a hydrogen-penetrating environment, and little deterioration in delayed fracture strength and rotational bending fatigue strength.

[0035]

As described above, the steel of the present invention is made of Ti
By the effective addition of and the optimization of manufacturing conditions, it has an excellent effect of providing a high-strength and long-life carburizing steel having excellent delayed fracture resistance.

[Brief description of drawings]

FIG. 1 is a diagram schematically showing the structure of a thrust life tester.

FIG. 2 is a diagram showing a test piece shape of a hydrogen cracking susceptibility test.

[Explanation of symbols]

1 Test piece holding frame 2 Thrust test piece 3 cage 4 3/8 inch steel ball 5 upper race 6 rotation axes 7 Lubricating oil

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁷ Identification code FI C23C 8/22 C23C 8/22 (56) References JP 10-17985 (JP, A) JP 5-263183 (JP , A) JP-A-11-201168 (JP, A) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00 C21D 6/00 C21D 8/00 C23C 8/22

Claims (6)

(57) [Claims] 1. A mass ratio of C: 0.10 to 0.40.
%, Si: 0.05 to 0.50%, Mn: 0.2 to 2.
0%, Ti: 0.05 to 0.20%, Al: 0.010
~ 0.50%, N: 0.0120% or less, O: 12 pp
An excellent delayed fracture resistance characterized by finely dispersing Ti carbide or Ti carbonitride having an average content of 70 nm or less and containing m or less and the balance Fe and unavoidable impurities in the steel. High strength and long life carburizing steel. 2. A mass ratio of C: C: 0.10 to 0.4.
0%, Si: 0.05 to 0.50%, Mn: 0.2 to
2.0%, Ti: 0.05 to 0.20%, Al: 0.0
10 to 0.50%, N: 0.0120% or less, O: 12
Containing ppm or less, further Ni: 0.1-2.0
%, Cr: 0.20 to 2.0%, Mo: 0.05 to 1.
It contains one or more selected from 0% and consists of the balance Fe and unavoidable impurities and has an average size of 7
A high-strength long-life carburizing steel having excellent delayed fracture resistance, which is characterized by finely dispersing 0 nm or less of Ti carbide and Ti carbonitride in the steel. 3. The high-strength, long-life carburizing steel according to claim 1 or 2, further comprising B: 0.0005 to 0.0050% by mass as an alloy component, and the balance Fe.
And unavoidable impurities, the average size is 70 nm
A high-strength long-life carburizing steel having excellent delayed fracture resistance, which is characterized by finely dispersing the following Ti carbide and Ti carbonitride in the steel. 4. A mass ratio of C: 0.10 to 0.40.
%, Si: 0.05 to 0.50%, Mn: 0.2 to 2.
0%, Ti: 0.05 to 0.20%, Al: 0.010
~ 0.50%, N: 0.0120% or less, O: 12 pp
A steel material containing m or less and consisting of balance Fe and unavoidable impurities is heated and rolled in a temperature range of 1200 to 1350 ° C., and further rolled or forged into a predetermined steel material or component at the same temperature or 800 to 1050 ° C. As a result, in the subsequent heat treatment step, T having an average size of 70 nm or less is obtained.
A method for producing a high-strength long-life carburizing steel having excellent delayed fracture resistance, which is characterized in that i carbide and Ti carbonitride are finely dispersed in the steel. 5. A mass ratio of C: 0.10 to 0.40.
%, Si: 0.05 to 0.50%, Mn: 0.2 to 2.
0%, Ti: 0.05 to 0.20%, Al: 0.010
~ 0.50%, N: 0.0120% or less, O: 12 pp
m or less, and further Ni: 0.1 to 2.0%, C
r: 0.20 to 2.0%, Mo: 0.05 to 1.0%, and one or more selected from the balance Fe.
And a steel material consisting of inevitable impurities from 1200 to 1350
Heating and rolling in the temperature range of ℃, and the same temperature or 8
By rolling or forging to a predetermined steel or component at a temperature of 00-1,050 ° C., a heat treatment step the size of the later
A method for producing a high-strength long-life carburizing steel having excellent delayed fracture resistance, characterized in that Ti carbide and Ti carbonitride having an average of 70 nm or less are finely dispersed in the steel. 6. The alloy component of the steel material in the method for producing a long-life bearing steel according to claim 4 or 5, further containing B: 0.0005 to 0.0050% in a mass ratio. , The balance Fe and the inevitable impurities are steel materials 1200 to 1
By heating and rolling to a temperature range of 350 ° C, and further rolling or forging into a predetermined steel material or component at the same temperature or a temperature of 800 to 1050 ° C, Ti carbide having an average size of 70 nm or less in the subsequent heat treatment step. A method for producing a high-strength long-life carburizing steel having excellent delayed fracture resistance, characterized in that Ti carbonitride is finely dispersed in the steel.